1. Field of the Invention
This invention relates generally to hybrid circuits for providing an interface between a pair of unbalanced unidirectional transmission paths and a balanced two wire bidirectional path and, more particularly, to such hybrid circuits which are direct coupled; i.e., transformerless.
2. Description of the Prior Art
Hybrid circuits are known which provide signal conversion between a balanced two wire bidirectional transmission path and a pair of unbalanced unidirectional transmission paths, the latter comprising a first path for carrying incoming electrical signals and a second path for carrying outgoing electrical signals. Hybrid circuits of this type are typically employed in telephone systems to provide a signal interface between a telephone carrier channel having the aforementioned pair of unidirectional transmission paths and a balanced bidirectional transmission path at either end of the carrier channel to which are connected the subscriber loop and the telephone instrument at one end and the central office equipment at the other end.
In the past, hybrid circuits have been conventionally implemented with transformers having a pair of loop terminals connected to the bidirectional signal path and terminals connected to the separate incoming and outgoing unidirectional signal paths. However, due to limitations inherent in transformer type hybrids, recent efforts have been directed to the design of transformerless hybrid circuits having substantially similar functional capability as transformer type hybrids. One such state of the art hybrid is shown and described in U.S. Pat. No. 3,849,609 and employs semiconductor current control current sources hereinafter referred to as current mirrors to convert incoming signals from one unidirectional transmission path to balanced signals on the bidirectional transmission path and to convert balanced signals originating in the bidirectional transmission path to outgoing signals on the other unidirectional transmission path. Circuits of this type possess the decided advantage of being monolithically integrable. This results in lower manufacturing costs, high reliability and small physical size.
U.S. Pat. No. 4,004,109 describes a transformerless hybrid which provides signal conversion between a balanced bidirectional transmission path and two unidirectional transmission paths, which suppresses longitudinal (common mode) signals originating in the bidirectional transmission path and which provides signal balance to suppress transmission of reflected echos from the incoming unidirectional transmission path to the outgoing unidirectional transmission path. Each embodiment disclosed includes a plurality of current mirror circuits, some of which are of complementary conductivity to the remaining current mirror circuits. Each current mirror circuit has a common terminal, one input terminal, at least one output terminal, and means for supplying currents to the output terminals which are proportional to the current at the input terminal.